Various filter and delay line devices have in the past been developed for use in such pulse processing applications as Pulse Compression Radar Systems which involve the transmission of a long coded pulse and the processing of the received echo to obtain a relatively narrow pulse. The increased detection capability of a long-pulse radar system is thus achieved while retaining the range-resolution capability of a narrow pulse system. Transmission of the long pulse permits a more efficient use of average power capability of the radar system and generation of high peak power signals is avoided. Additionally, the radar is less vulnerable to interfering signals that differ from the coded transmitted signal.
Systems utilizing this technique are described in detail in Chapter 20 of a book entitled, "Radar Handbook" published by the McGraw Hill Book Company in 1970 and edited by Merrill I. Skolnik of the U.S. Naval Research Laboratory. As pointed out therein, the pulse compression ratio is normally defined in the art as the ratio of width of the expanded pulse to that of the compressed pulse. The pulse-compression ratio is also equal to the product of the time duration and the spectral bandwidth (time-bandwidth product) of the transmitted signal. In passive systems a matched filter approach is used for the expansion and compression of the pulse. For example, filters may be used which are conjugates of each other for the expansion and compression. A filter is also matched to a signal if the signal is the time inverse of the filters's response to a unit pulse.
The best known and most widely used form of pulse compression in radar system is linear FM or "chirp". A linearly frequency modulated pulse is transmitted, producing a quadratic phase versus time history, and the received pulse is conventionally compressed by passing the signal through a dispersive delay line, usually after conversion to intermediate frequencies. The chirp waveform which is originally transmitted may be generated actively by sweeping an oscillator or passively by pulsing a dispersive delay line with a burst of carrier signal at intermediate frequencies. In the passive technique, the signal is usually but not necessarily compressed with the same dispersive network used to generate the waveform. Other usable coding includes phase coding and the even older forms of amplitude modulation such as transmisstion of a simple ramp signal.
On pages 20-35 and 20-36 of the Skolnik "Radar Handbook" there is described an optical correlator which uses optical techniques to provide matched filters for such pulse compression radar systems. The system shown in FIG. 31, thereof, for example, uses collimated monochromatic light applied orthogonally through a transparent ultrasonic light modulator. The received electrical signal at an IF frequency is applied to the transducer of the ULM where it is converted to an ultrasonic wave which propagates through the ULM and is absorbed at the opposite end. The collimated light incident upon the ULM becomes spacially modulated by the ultrasonic wave. A stationary reference mask consists of a grating pattern which corresponds to the coded signal waveform. Correlation occurs when the modulated signal caused by the received signal coincides with the pattern on the referenced mask. The compressed pulse is obtained at the output of a photodetector scanning the correlation mask. The minimum output pulse width is necessarily determined by this scanning time and by the transit time of the acoustic wave through the ULM orthogonally to the incident light pulse rather than by the duration of the light pulse. The general analytic theory of diffraction of light by ultrasonic waves intersecting the light waves orthogonally has been discussed at pages 593 through 610 of a book published in 1970 by the Pergamon Press entitled "Principles of Optics" by M. Born and E. Wolf. Neither of the above referenced books, however, discusses the phenomenon of colinear acousto-optical diffraction as distinguished from orthogonal acousto-optic diffraction.
However, in the heretofore unrelated art of optical filters, an acousto-optic tunable filter using colinear acousto-optic diffraction has recently been developed and reported by S. E. Harris and R. W. Wallace as described beginning at page 744 of Volume 59, No. 6 of the Journal of The Optical Society of America in June 1969. Further details were given by S. E. Harris and S. T. K. Nieh at pages 223-225 of Volume 17, No. 5 of "Applied Physics Letters", Sept. 1, 1970 and by the same authors together with D. K. Winslow pages 325 and 326 of Volume 15, No. 10 of "Applied Physics Letters", Nov. 15, 1969. This tunable filter employed a phenomenon originally described in detail by R. W. Dixon (IEEE, J. Quantum Electron, Q.E.-3 85, 1967). Dixon noted that in an appropriately oriented crystal, an incident optical beam of one polarization is deflected into the orthogonal polarization during its interaction with the colinearly propagating acoustic beam. In order for this phenomenon to occur, the active crystal medium must possess a non-zero element of the photo-elastic tensor appropriate to the interaction. The appropriate photo-elastic constant depends on such factors as crystal symmetry and whether a longitudinal or transverse acoustic wave is employed. Moreover, for the coupling to be effective along the whole interaction length, it is necessary that the optical and acoustic waves be appropriately phase matched. For a given acoustic frequency the phase matching condition is satisfied over a relatively narrow range of optical wavelength. Hence, only light in this wavelength range will be scattered from the original input polarization state to the polarization state orthogonal thereto.
Harris and Wallace proposed an electronically tunable optical filter using this phenomenon. Their basic idea was to utilize the linear acousto-optic diffraction in an optically anisotropic medium in such a fashion that by electronically changing the frequency of a pure C. W. sinusoidal driving acoustic wave, changes were produced in the band of optical frequencies that the filter passed. In their paper they give the specific details for a filter using a crystal of LiNbO.sub.3. The Harris and Nieh paper described a filter using a crystal of CaMoO.sub.4. Both papers note that when an acoustic wave travels in such a crystal, the strain induced change of the refractive index of the medium may diffract the light beam that is incident on the medium. In an isotropic medium, the polarization of the diffracted light is unchanged and the diffraction is particularly strong when the light is incident at the Bragg angle. In an anisotropic medium, for certain orientation, light may be diffracted from one polarization to another. In this case, the condition for interaction between the acoustic wave and the light wave is that the sum of the k vectors of the incident light and the acoustic wave equal the k vector of the orthogonally polarized diffracted wave. In their filter a crystal orientation is chosen such that an incident optical signal of one polarization is diffracted into the orthogonal polarization by a colinearly propagating acoustic beam. For a given acoustic frequency only a small range of optical frequencies will satisfy the k vector matching condition and only this small range of frequencies will be cumulatively diffracted into the orthogonal polarization. If the acoustic frequency is changed, the band of optical frequencies which the filter will pass is changed.
In the Harris device the crystal is preceded by a polarizer through which light to be filtered is passed before entering the crystal and is followed by an analyzer having its polarization axis orthogonal to that of the polarizer so that only those frequency components of the beam which have been diffracted orthogonally in the crystal will pass through the analyzer. An electronically driven acoustic transducer supplies to the crystal a constant radio frequency signal of known preselected fixed single frequency to determine the passband of the filter with respect to an optical light beam of unknown mixed and/or variable frequency components. The basic phenomenon in the Harris device is utilized herein in a different manner and for a different purpose in other system combinations.